(1) Field of the Invention
This disclosure relates to a composition comprising a silane, and a system of peroxides and a process of crosslinking thermoplastic polymers with said composition. The disclosure further relates to the moisture crosslinked thermoplastic polymers resulting from the process and articles made therefrom.
(2) Description of Related Art
For many applications, e.g., wire and cable insulation, weatherstripping, fibers, seals, gaskets, foams, footware, flexible tubing, pipes, bellows, tapes, etc., certain selected properties (e.g. tensile strength, compression set, thermal and chemical resistance) of articles manufactured from one or more thermoplastic polymers can be enhanced by introducing chemical linkages between the polymeric molecular chains which constitute the polymer, during or specifically following, the shaping or molding process. These chemical linkages between different polymeric molecular chains are commonly referred to as “crosslinks”. Crosslinks can be introduced between different molecular chains of a thermoplastic polymer by a number of mechanisms, one of which is to graft to the individual polymer backbones or chains that constitute the bulk polymer with a chemically reactive compound in such a manner that the grafted compound on one backbone may subsequently react with a similar grafted compound on another backbone to form the crosslink. Exemplary of this process is the “silane crosslinking” process.
The silane crosslinking process employs a silane-containing thermoplastic polymer that crosslinks when exposed to moisture. Silanes can be grafted onto a suitable thermoplastic polymer by the use of a suitable quantity of free radical initiator, either before or during a shaping or molding operation. Additional ingredients such as stabilizers, pigments, fillers, catalysts, processing aids etc., may also be included in the mixture.
When using practicing silane cross-linking for thermoplastic polymer, a compromise must be made between grafting efficiency and processing efficiency, such as extrusion rate and run times. The formation of a crosslinkable material by this means is, however, difficult to carry out since it requires critical control of the process. If the free radical initiator, for example, reacts too quickly with the thermoplastic polymer, then the thermoplastic polymer may partially crosslink and solidify in the processing apparatus, for example an extruder, with consequent difficulties in achieving consistent and good quality products and in avoiding delays involved in removing the partially crosslinked product from the processing equipment.
It has been observed that gel formation, screw-build up and scorching may result when using highly reactive silane cross-linking blends. This gel formation is particularly significant for processes using conditions and processing equipment that impose severe melting and mixing conditions leading to high shearing stresses in the thermoplastic polymer. These problems generally arise due to early and eventually complete activation of the free radical initiator during the initial melting and homogenization process. The prior art has dealt with these problems by using less reactive silane cross-linking blends but this approach can diminish the grafting efficiency of the crosslinkable thermoplastic polymers.
The silane compositions of the prior art can also generate volatile organic compounds in amounts that are a potential fire or explosion hazard and may be deleterious to the environment.
Thus, there remains a need for a means of crosslinking polyolefins and other silane crosslinkable thermoplastic polymers under reactive mechanical-working conditions using silane crosslinkers and free radical initiators while minimizing such aforenoted problems as gel formation, screw-buildup and/or scorching, fire or explosion hazards while maintaining a high level of grafting efficiency.